Biological Sciences Division Research Highlights

Electron migration via protein trafficking tightly regulated by cell

Results: When it comes to transporting a cell's
valuable electrons, the metal-reducing microbe Shewanella oneidensis only trusts
stable, mature proteins, according to scientists at Pacific Northwest National
Laboratory. Immature proteins are degraded before they can take up the task,
suggesting that protein trafficking to the outer membrane is tightly regulated.

These results mark another step toward understanding the cellular mechanisms that enable a bacterial protein-in this case, the cytochrome MtrC-to transfer electrons to minerals in soil,
sediment, and subsurface materials. This new information contributes to understanding
protein stability and electron transfer between cells and minerals, which is
important for applications in synthetic biology such as biofuel production. The
results were published in the journal Biochemistry.

Why it matters: Electron transfer by MtrC, an outer
membrane cytochrome on S. oneidensis,
can stabilize contaminants, mitigating their impact on the population and
environment. However, scientists believe that gaining insight into the electron
transfer mechanisms could also play a role in directing the bacterium toward
biofuel production.

"Our goal is to define the role of these cytochromes in the
metabolic switching between different terminal electron acceptors," said Dr.
Thomas Squier, a PNNL biochemist and senior author of the publication. "The
long-term goal is to understand the stability and targeting mechanisms
important to synthetic biology applications involving, for example, chemical
sensing between living cells and electronic detectors as well as the
development of biofuel cells."

These findings don't just relate to Shewanella, though it was in this microbe where MtrC was first
seen. They also apply to many other bacteria, such as E. coli, notes Squier.

"This research ties very well into looking at and
understanding microbial communities as a whole," he added.

Structures of biarsenic fluorophores, CrAsH-EDT2 and FlAsH-EDT2. FlAsH-EDT2 represents a fluorescein scaffold derivatized with two As(III) groups (4′,5′-bis(1,3,2-dithioarsolan-2-yl)fluorescein). Each arsenic is capped with an ethanedithiol (EDT). Similarly, CrAsH-EDT2 represents 6-carboxy-FlAsH-EDT2.

Methods: Measuring MtrC's environmental
stability requires the ability to differentiate an immature protein from a mature
protein after it is secreted and assembled on Shewanella's outer membrane. To do this, the
scientists constructed complementary fluorescent probes to label MtrC. The highly
charged carboxy-FlAsH (CrAsH) probe selectively labels mature MtrC only on the
outer cell membrane, while the cell-permeable Fluorescein
Arsenical
Helix
(FlAsH) probe labels all MtrC, including immature proteins within the cell.

Acknowledgments: This work was supported by the U.S. Department
of Energy Office of Biological and Environmental Research Genome Science
Program. The research team includes Yijia Xiong, Thomas C. Squier, Baowei Chen,
Liang Shi, James K. Fredrickson, and Diana J. Bigelow, PNNL.